Coticules and BBW's both contain Spessartine Garnets.
They are in both cases duodecahedron shaped crystals. Duodecahedron are spatial objects with 12 facets. These facets are mostly pentagons.
Here a model of a duodecahedron:
Coticules contain a high concentration of small garnets (5 to 15 micron in diameter): up to 40% of the entire volume of the rock.
BBWs contain a lower concentration of wider garnets (10 to 25 micron in diameter): up to 25% of the entire volume of the rock.
Those are the figures for the fastest layers. In slower Coticules, garnet content may drop to 20%, or even less in a very slow layer like "La Veine aux Clous", which is usually discarded for that reason.
Within the blue schiste, garnet content can vary between that 25% and.... zero%. That's right: there is blue rock that contains no spessartine at all. That's why only the blue rock with garnet content is commercialized as "Belgian Blue Whetstone". The rest is used for other purposes, there are even houses built with blue stone. It is mostly the blue schiste that borders on the Coticule layers, that contains garnets.
Talking about grit rating is completely meaningless. If we compare the size of the garnets to the particle sizes of synthetic hones, then Coticules have a "grit size" of 15 to 5 microns, or if you want that in approximate mesh numbers: 1000 to 3000 grit. For the BBW: 600 to 2000 grit.
So, why is it not stated as such?
Firstly because it is irrelevant, and secondly, because we live in a world that is dominated by synthetic hones. And the majority of people in that world likes to believe that:
a car with more HP is better than one with less HP, as photocamera with more megapixels shoots better pictures than one with less megapixels, a guitar amp of 100watt is better than one of 30watt, a guy with a dick of 15cm is a better lover than one with a dick of 10cm. Do I need to continue?
In other words, Ardennes would be nuts to put on their website "natural whetstones with grit size of 1K". Instead, they state "8K". A few years ago, 8K in synthetic hones was considered sufficient for shaving. A Coticule edge is sufficient for shaving. So, there's the 8K logic. I think Coticules are better than that, but that's just me.
The BBW stones used to be sold in the past as "Pierres Lorraine" or "Rouge de Salme". It was a cheaper whetstone, which it still is, because it's easier to extract and there's more available. Large bench stones are no problem. It was popular for sharpening all kinds of cutlery. BBW's are considerably slower than Coticules. That has been probably considered in the past as a disadvantage, notably for razors, with their very hard steel and restrictions againt the use of high pressure. In the old days, barbers were not talking about "bevel setting", "intermediate grits" and "edge finishing". Instead, they were honing on a stone till they felt they'd done enough, and then they went stropping. Even J.J.Perret, as early as 1770, talks about strop dressing. He mentions: pumice, terracotta, ttone dust, rouge, amaril, and a couple other known abrasives. That tells me they were using their hone for what I now call "bevel correction". I don't think that it is a coincidence that the grading system of the ancient Coticule companies favored the fastest layers. In that respect, the "Rouge de Salme" was a lesser stone.
I think Ardennes positioned the BBW in the 4-6K region for that reason: slightly under Coticules, their top op the shelve product. And it seems in accordane with the scientific data about the garnet size.
But the diameter of its particles is not the only contributing factor in the capabilities of a whetsone. There might be more going on than just a story of garnets, but nonetheless, the speed of a hone relates to how deep it scratches the steel. And even if the depth of those scratches is not the only contributing factor, it is indubitably one of the key variables in the keenness that can be attained with a particular hone. Slow hones go keener, unless they are so dead slow that they can't make up for what little wear they cause on the tip the edge that is pushed over their surface. In that case I call them polishers. Coticules on water are flirting with that threshold, and BBW's even more.
As far as hardness is supposed to be an indicator of something, I have not been able to find any truth in such a statement. I've tested soft Coticules that were slow, hard Coticules that were fast, and vice versa. I've found any of my finishing classifications (brisk, engaging and mellow) accross the entire spectrum of Coticule hardness. I think there is too much fantasy taken for fact.
Kind regards,
Bart.
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